The present invention generally relates to subterranean well fracturing operations and, in representatively illustrated embodiments thereof, more particularly relates to specially designed expandable fracture plug seat structures and associated apparatus for operatively supporting them downhole and selectively permitting and precluding expansion thereof.
In subterranean well stimulation, the ability to perforate multiple zones in a single well and then fracture each zone independently, (typically referred to as “zone” fracturing), has desirably increased access to potential hydrocarbon reserves. Many gas wells are drilled with zone fracturing planned at the well's inception. Zone fracturing helps stimulate the well by creating conduits from the formation for the hydrocarbons to reach the well. A well drilled with planned fracturing zones will be equipped with a string of piping below the cemented casing portion of the well. The string is segmented with packing elements, fracture plugs and fracture plug seat assemblies to isolate zones. A fracture plug, such as a ball or other suitably shaped structure (hereinafter referred to collectively as a “ball”) is dropped or pumped down the well and seats on the fracture plug seat assembly, thereby isolating pressure from above.
In order to progressively fracture successive subterranean zones along the length of the wellbore it is necessary to construct the ball seat so that its annular shape is diametrically expandable to permit one or more fracture balls to be forced therethrough on their way to expandable plug seats further downhole to sealingly seat on these lower seats. It is further necessary to selectively preclude diametrical expansion of the seats to permit this sealing engagement between a fracture ball and the seat.
Previously proposed expandable fracture ball seats of this general type have been subject to well known problems, limitations and disadvantages. For example, in order to permit the necessary diametrical expansion of a ball seat it is typically necessary to form one or more radial slits therein which widen as the fracture ball passes through the seat. These necessarily widened slits have proven to be susceptible to having well debris lodged therein which can undesirably prevent proper complete closure of the gaps, when the seat returns to its smaller diameter relaxed position, thereby denigrating the requisite sealing capability of the seat when it is called upon to be sealingly engaged by a fracture ball plug (i.e., when the ball is acting as a plug) and prevent its passage through the circular seat opening.
Additionally, during the high pressure injection of frac slurry into a perforated downhole formation, the plug seat is subject to an abrasive blasting effect of the slurry. In conventionally designed plug seats this causes erosion of the seats, thereby lessening their plug sealing ability. Moreover, conventionally constructed plug seats, due to the driving pressure exerted on the ball plugs, may create stress concentrations on the balls sufficient to deform them and thereby substantially reduce the sealing capability of the associated ball seat.
As can be seen from the foregoing, a need exists for an improved expandable fracture ball seat structure which eliminates or at least reduces the aforementioned problems, limitations and disadvantages associated with previously proposed expandable fracture plug seats as generally described above. It is to this need that the present invention is primarily directed.